This invention relates generally to the field of inertial navigation instruments. More specifically, the invention relates to methods for determining orientation of an inertial navigation instrument disposed in a wellbore drilled through earth formations
Directional wellbores are drilled through earth formations along a selected trajectory. The selected trajectory deviates from vertical at selected inclination angles and at selected azimuth (geographic reference) directions along the length of the wellbore. It is necessary to measure the inclination and azimuth of the wellbore during drilling to determine whether the selected trajectory is being maintained. Typically, a so-called directional survey is performed to measure the inclination and azimuth at selected positions along the wellbore. A directional survey is often performed by inserting a survey instrument into a drilling tool assembly, and moving the drilling tool assembly into or out of the wellbore. At selected intervals, usually about every 90 feet (30 meters), the drilling tool assembly having the instrument therein is stopped so that a survey measurement can be made. The survey instrument includes sensors which generate measurements corresponding to the instrument orientation with respect to the geographic reference and to earth""s gravity. Azimuth and inclination are determined from the instrument orientation. The measurements of azimuth and inclination at each survey location are then entered into any one of a number of well known calculation methods to determine the wellbore trajectory along its entire length.
Many types of surveying instruments are known in the art for measuring the inclination and azimuth of the wellbore. One type of instrument uses gyroscopes in one form or another to establish the geographic reference with respect to the survey instrument, and uses accelerometers to determine the orientation of the instrument with respect to the earth""s gravity (vertical). Types of gyroscope-based instruments used for wellbore surveying are described, for example, in U.S. Pat. No. 4,454,756 issued to Sharp et al. and in U.S. Pat. No. 4,987,684 issued to Andreas et al. As is known in the art, certain gyroscope types generate a signal output which is related to the angular velocity of the gyroscope about its sensitive axis. Measurements of the angular velocity can be integrated to provide rotational displacement of the gyroscope about its axis. Rotational displacement can be used to determine rotational orientation of the instrument.
A particular type of gyroscope-based survey instrument is known in the art as a xe2x80x9cstrapdownxe2x80x9d instrument. Strapdown instruments have gyroscopes which are fixed to the instrument housing or frame, and determine the geographic reference by measuring components, usually along three orthogonal axes, of the earth""s rotation about its axis. The orientation of the earth""s rotation vector with respect to the instrument is used in conjunction with the orientation of the instrument with respect to gravity to determine the wellbore azimuth and inclination.
Measuring the components of the earth""s rotation is difficult because the rotation rate of the earth is relatively small. Typically, it is necessary in wellbore surveying to hold the instrument steady (substantially motionless) for at least a minimum measurement time to enable determination of the earth""s rotation component in each of the gyroscope signals. As is known in the art, the minimum measurement time is related to, among other factors, the amount of noise in the output of the particular type of gyroscope used. Frequently, gyroscope instruments are subject to movement during a survey measurement. Such movement can be caused by, among other factors, careless operation of the drilling tool assembly, and motion of the drilling tool assembly because of ocean waves where the wellbore is drilled from a floating vessel in the ocean. When the survey instrument is moved during a measurement period, the instrument may be subject to momentary high angular acceleration. Such angular acceleration can make survey computations inaccurate or useless. However, the opportunity to determine the wellbore inclination and azimuth at the particular location may be very important to maintaining the wellbore along its intended trajectory.
What is needed is a method for calculating wellbore surveys from gyroscopic measurements which can take account of, and at least partially correct for, instrument movement during a survey measurement.
The invention is a method for calculating a gyroscope survey. The method includes measuring outputs of gyroscopes and accelerometers in a survey instrument. An instrument rotational movement is determined from the outputs of the gyroscopes. If the rotational movement exceeds a predetermined threshold, an instrument attitude is calculated from the gyroscope measurements, using the most recent alignment measurement to initialize the attitude measurement. Instrument alignment is calculated from the output of the accelerometers and gyroscopes when the threshold is not exceeded.
In one embodiment of the invention, the instrument alignment is calculated using Kalman filtering. In one embodiment, the rotation of the instrument is determined by measuring a root sum square average of the gyroscope outputs.
In a particular application of the method, the azimuth and inclination of a wellbore are determined at selected positions along the wellbore from the alignment and/or attitude measurements. The wellbore azimuth and inclination are used in one embodiment to calculate the wellbore trajectory along its length. One embodiment of the trajectory calculation includes performing a minimum curvature calculation.